There are several tracking devices on the market. One option is a GPS tracker using a sim card to send out location details. Another option is an RF beacon which works on UHF. FMKit makes one. The problem with this tracker is that it will probably interfere with UHF radios.

Something I need to check on, is tracking capabilities of the EZUHF radio modules, which I use. I know their receivers are also sending telemetry data back to the Tx (RSSI for starters). It might be possible to use this in some way. I'll be back with more info later.

After more reading about the FMKit option, I see they have a VOX version which only starts transmitting once it can't pick up motor and wind noise anymore (in other words, when the plane is on the ground). It uses a microphone to listen for noise or the lack of. Clever!

I wonder how small and light it would be if you take it out of the case. Then do the silicone/hot glue thing and wrap it in foam or something.You're right, it's probably the best option for lost aircraft. Nothing like a set of GPS coords to lead you directly to the crash site.